Automatic tracking system utilizing coded scan rate



Dec. 31, 1968 P. M. PlFl-:R 3,419,867

. AUTOMATIC TRACKING SYSTEM UTILIZING CODED SCAN RATE I INVENTOR.

P57516 f7. P/ F52 Dec. 3l, 1968 P. M. P11-ER 3,419,367

AUTOMATIC TRACKING SYSTEM UTILIZING CODED SCAN RATE Original Filed Oct.18. 1966 s//wso/z F/v fo Sheet 2 of 2 I N VEN T OR. pfff/9 P/Ff/- UnitedStates Patent O 3,419,867 AUTOMATIC TRACKING SYSTEM UTILIZING CODED SCANRATE Peter M. Pifer, Atlanta, Ga., assignor to Scientific-Atlanta, Inc.,Atlanta, Ga., a corporation of Georgia Continuation of application Ser.No. 587,608, Oct. 18, 1966. This application Feb. 21, 1968, Ser. No.707,335

6 Claims. (Cl. 343-113) ABSTRACT F THE DISCLOSURE Disclosed in thisapplication is an antenna tracking system in which the antenna beam ismodulated as a measure of the deviation of the line-of-sight to thetarget from the boresight axis of the antenna in such a continuallychanging coded manner as to make that modulation distinguishable fromother modulation, including extraneous signals, present on the incomingsignal received by the antenna. The antenna beam is moved in accordancewith a coded scan control signal whose scan rate varies. That is, therate of scan is coded. The same coded control signal is correlated withthe signal that is received by the antenna, to distinguish the deviationmeasuring signal in order that the signal only is employed to deriveantenna pointing-error signals which are therefore free of the othermodulation received by the antenna.

This is a continuation of application Ser. No. 587,608, filed Oct. 18,1966, and now abandoned.

This invention has application in any tracking antenna systemincorporating electronic means of scanning, switching, or lobing theantenna beam to impart a modulation to the signal received from thetarget, the modulation constituting a measure of the deviation of theline-of-sight to the target from the boresight axis of the antenna. Inparticular, the purpose of the invention is to make the modulation thusimparted distinguishable from other modulation present on the incomingsignal received by the antenna, and to detect the scan-generatedmodulation only so as to derive antenna pointing-error signals that arenot affected by such other modulation.

Further, this invention relates to an improvement of the inventiondisclosed lby L. Clayton, Jr., in the patent application Scan AutomaticTracking System Utilizing Coded Scan Sequence, Ser. No. 719,265, filedApr. 5, 1968, and which is a continuation of Ser. No. 587,528, tiledOct. 18, 1966, now abandoned.

Tracking antenna systems can be classified broadly into the followingcategories:

(a) Mechanically scanning or lobing antenna systems, such as the widelyused conical-scan type, which impart boresight-error modulation to thesignal, employ a singlechannel receiver, and derive pointing-errorsignals from the detected envelope of the modulated signal.

(b) Monopulse or simultaneous lobing systems which have a nonscanningantenna, employ a multiplechannel receiver to process the principalsignal and one or two error signals provided by multiple ports of theantenna, and derive pointing-error signals by synchronous detection ofthe error-channel IF signals using the principal-channel IF signal as areference.

(c) Electrically scanning or lobing antenna systems which impart aboresight-error modulation to the signal, employ a single-channelreceiver, and derive pointingerror signals from the detected envelope ofthe modulated signal. Such a system may consist of an antenna of any ofthe types suitable for use in a monopulse system, an electronic scanningdevice, and a single-channel receiver of any of the types suitable foruse in a conical-scan ice system. The electronic scanning device mayincorporate means of switching, amplitude modulating, or phase shiftingthe signals from the antenna error ports and combining them with theprincipal signal so as to produce a composite signal similar to thatproduced by mechanical scanning or lobing.

Mechanically scanning antennas are subject to wear, require maintenance,and are normally limited to scan rates of 30/sec. or lower. Their mostserious limitation in many applications is that the boresight-errormodulation they produce is indistinguishable from modulation presentupon the incoming signal due to multipath propagation interferencephenomena, spinning or other motion of the target, or intentionaljamming. Such modulation having frequency components near the scan ratecauses spurious pointing-error signals which result in degradedperformance or loss of track.

Monopulse systems overcome these ditliculties, ybut the requiredmultichannel receiver is complex and expensive, and is difficult toadjust and maintain because of the necessity for precise phase stabilityand automatic-gaincontrol tracking among channels.

Electrically scanning systems overcome the mechanical disadvantages ofscanning antennas while retaining the simplicity of a single-channelreceiver. They suffer the same limitation of sensitivity to spuriousmodulation. Their performance is generally superior to that ofmechanical systems, however, because the scan rate can be made highcompared to the Ipredominant frequencies encountered from spinmodulation and multipath effects. Nevertheless, the scan rate is oftenrequired to be lower than information-'bearing modulation frequenciespresent in the signal, and the complex periodic modulation produced bytarget spin can have significant harmonic content in the region of a fewhundred hertz. Even when not so severe as to cause loss of track, beatphenomena cause cyclic tracking errors.

It is a primary object of this invention to provide a scanning patternwhich is coded, and thus provides coding of the error-signal modulationto make it noncoherently related to modulation which might be present onthe incoming signal.

A brief description of an illustrative embodiment of the invention willnow be given:

The automatic tracking antenna shown lcould be any antenna usingelectronic means of generating an amplitude-modulated RF signal whoseamplitude modulation is related to the oit-boresight angle. The RFsignal is normally (but not necessarily) amplified and detected by an AMreceiver. The detected signal is in an AC form, containing error signalscorrelated with the coded scan drive signal, and also containing othersignals corresponding to the amplitude modulation on the incoming RFsignal. The AC error signal is converted to a DC error signal in across-correlation detector which uses as reference the scan drivesignal. The detector will be nonresponsive to signals other than thosethat have significant correlation with the scan drive signal.

The preferred approach is to cause the rate of repetition of the scancycle to vary. That is, the scan rate is coded rather than the scansequence, as is done in the before-mentioned copending application. Thescan code generator varies the scan frequency rapidly in a cyclic orrandom manner, executing a pattern of scan frequency variation in a timethat is short in comparison with the response time of the antenna servo.

Another object of this invention is to make an electrically scanningtracking antenna system almost completely immune to disturbance fromamplitude modulation of the incoming signal, without substantialincrease in cost or complexity.

A further object of this invention is the use of a coded scan rate in anautomatic tracking system, in combination with a pointing-error detectorthat is nonresponsive to signals other than those that have significantcorrelation with the scan drive signal.

Still other objects will be in part apparent and in part pointed outspecifically hereinafter in connection with the drawings that follow,and in which:

FIGURE l is a block diagram of an illustrative ernbodiment of theinvention;

FIGURE 2 is a graph illustrating one type of coded waveform employed inthe invention;

FIGURE 3 is a block diagram illustrating one embodiment for generatingthe waveform shown in FIGURE 2.

FIGURE 4 is a graph illustrating a waveform which occurs within thediagram of FIGURE 3; and

FIGURES 5, 6, and 7 are graphs which illustrate various alternativecoding waveforms which may be employed in the invention rather than thatshown in FIGURE 2.

Referring to FIGURE 1 there is shown an overall block diagram of thesystem. The tracking antenna is indicated at 10, the radio frequencysignal being received by this antenna. The magnitude of the modulationdeveloped by the tracking antenna corresponds to the deviation of theline-of-sight to the target from the boresight axis of the antenna.

The signal is applied to receiver 12 where the modulation is detectedand then applied to cross-correlation detector 14 via line 16. Alsoapplied to cross-correlation detector 14 over line 18 is a coded signalfrom scan rate code generator 22. The coded signal is also applied overline to antenna 10 to vary the scan rate of the antenna beam in apseudo-random manner as will be described in more detail hereinafter.

The output of the cross-correlation detector is a direct current errorsignal which is applied to antenna servomechanism 24 over line 26. Themagnitude of this error signal depends ideally only on the deviation ofthe line-ofsight to the target from the boresight axis of the antenna10. However, the error signal could be affected by various extraneoussources such as multipath propagation interference phenomena, spinningor other motion of the target, or intentional jamming. The manner bywhich erroneous components from these extraneous sources are removedfrom the error signal on line 16 will be described in more detailhereinafter. The error signal on line 26 may control antennaservo-mechanism 24 so that the deviation of the target from theboresight axis is minimized by appropriate means as indicated at 28.

Having now described in general terms the structure required for theaccomplishment of the objects of this invention a description ofoperation thereof will now be given. Reference should be made to FIGURE2 which graphically illustrates one method of changing the outputfrequency of code generator 22 with time. Other methods are shown inFIGURES 5, 6, and 7 and will be described hereinafter. The beam scanrate of antenna 10 is the same as frequency of the coded signal and,therefore, it changes with time in the same manner as the coded signal.

As indicated in FIGURE 2 the change of output frequency is periodic. Inlight of this periodicity the change of output frequency ispseudo-random as opposed to purely random. However, it would, of course,be within the scope of one having ordinary skill in this art to employ apure random signal generator for scan code generator 22 rather than thepseudo-random generator which is described in this embodiment of theinvention.

The output signal from code generator 22 thus corresponds to the signalshown in FIGURE 2. This signal is applied to automatic tracking antenna10 to change the scan rate thereof with respect to time. By thischanging the scan rate, the frequency of the off-boresight deviationamplitude modulation introduced on the signal received from the targetis accordingly changed. As stated hereinafter, various sources couldintroduce extraneous amplitude modulation which can impair the accuracyof the error signal or deviation measurement on line 26. However, due tothe pseudcxrandom nature of the frequency change imparted to theoff-boresight modulation on the received signal by scan code generator22, there is very little or no correspondence between the imparteddeviation frequency and the frequencies of the extraneous sources. Thus,if the frequency of a particular extraneous source is 300 hertz, theimparted frequency is 300 hertz for a relatively short time, compared tothe total period shown in FIGURE 2, and thus the output of crosscorrelation detector 14 reflects the 300 hertz of the extraneous sourcefor only a small portion of the total period indicated in FIGURE 2.Hence, the effect of the extraneous source is minimized.

The amplitude demodulated signal on line 16 contains frequencycomponents due to (l) the off-boresight deviation frequency changesimparted by the tracking antenna 10 and the code generator 22 and (2)various undesired signals introduced by different sources extraneous tothe tracking system. The output of cross-correlation detector line 26 ismaximized when the frequency of the signals on lines 16 and 18 isidentical. Thus, the smaller the deviaiton of the frequency of thesignal on line 16 from the frequency of the signal on line 18, thegreater the signal appearing on line 26. Thus, the frequency cornponentof the demodulated signal on line 16 corresponding to the off-boresightdeviation will result in a maximized direct current error signal on line26 whereas the frequency components of the demodulated signal on line 16due to the various extraneous sources will cause minimal output on line26 due to the noncorrelation in frequency of these extraneous signals tothe frequency of the signal on line 18. The optimum frequencies of thesignal on line 18 could be determined by making a statistical study ofthe expected frequencies of the various possible extraneous sources andthen choosing the frequencies of the signal on line 18 to correlate aslittle as possible with these expected frequencies. Typically, thefrequencies of these extraneous sources are from 0 to 30 hertz. Sincethe frequency of the scan rate preferably varies between 200 and 400`hertz, the fundamental frequency of a 30-hertz interfering source iseffectively removed. However, since the harmonic frequencies of the30-hertz source and other extraneous sources may occur between 200 and400 hertz, this invention insures that these additional signals willintroduce little, if any, error into the system.

In FIGURE 3 there is shown in block diagram form a preferred arrangementfor deriving the signals occurring on lines 18 and 20 of FIGURE l. Thusthe output of generator 42 as shown in FIGURE 4 is applied to voltagecontrolled oscillator 36, the frequency of the output of which varies inaccordance with the amplitude of the signal applied thereto. Hence,referring to FIGURES 2 and 4, the frequency of the output of oscillator36 for portion 30 of the generator 42 output signal would be somefrequency f1; whereas the frequency of the output of oscillator 36 wouldbe some frequency f2 for portion 32 of the code generator output signaland so forth. The output signal from oscillator 36 is applied to driveramplifiers 38 and 40` which are respectively connected to lines 18 and20 of FIGURE 2.

Reference should now be made to FIGURES 5, 6, and 7 which show variouswaveforms representing changes of frequency with respect to time of thesignal applied from scan rate code generator 22 of FIGURE l. Thewaveforms shown in FIGURE 2 correspond to a step change in the frequencywith respect to time, Whereas the change in frequency with respect totime in FIGURE 5 is sinusoidal, FIGURE 6 is triangular, and FIGURE 7 isalso triangular, the width of the triangular waveform in FIGURE 7varying while in FIGURE 6 the width remains constant.

The waveform of FIGURE 5 would naturally be the easiest to implementsince the signal output from code generator 22 in FIGURE 3 is merelysinusoidal thereby causing the sinusoidal change in frequency at theoutput of oscillator 36, as shown in FIGURE 5. Thus, the waveform ofFIGURE 5 is preferred when simplicity of construction and economy ofcost are the main prerequisites of the system.

However, a more complicated waveform, such as that of FIGURE 7, is quiteoften preferred. For instance, the operation of a tracking system in thepresence of a jamming signal `would require the use of waveform shown inFIGURE 7 as opposed to that shown in FIGURE 5. Although this wouldaffect to some extent the complexity of the equipment, this addedcomplexity is justified in light of the need to overcome the effect ofthe jamming signal.

Still numerous modifications of the invention would become apparent toone of ordinary skill in the art upon reading the foregoing disclosure.During such a reading, it would be evident that this invention hasprovided unique apparatus `for accomplishing the objectives andadvantages herein stated. It is to be understood, however, that thisforegoing disclosure is to be considered exemplary and not limitative,the scope of the invention being defined by the following claims.

I claim:

1. Apparatus for reducing the effects of various extraneous lmodulationof a signal received from a target in tracking systems which includemeans for scanning, switching or lobing an antenna beam and therebyimparting deviation modulation to the signal received from the targetwhich is a measurement of the deviation of the line-o-sight to thetarget from the boresight axis of the antenna, said apparatuscomprising:

antenna means for receiving said signal and imparting said deviationmodulation, l means for im-parting a frequency change to said deviationmodulation,

and detector means responsive to 'both the modulation on said receivedsignal and to said frequency change for developing said deviationmeasurement, whereby the correspondence between said deviationmeasurement and the imparted deviation modulation is maximized due tothe correlation of said imparted frequency change and said deviationmodulation and the said effects of Various extraneous modulation areminimized due to noncorrelation with said imparted frequency change.

2. Apparatus for reducing the effects of Various eX- traneous modulationof a signal received from a target in tracking systems which include-means for scanning, switching or lobing an antenna beam :and therebyimparting deviation modulation to the signal received from the targetwhich is a measurement of the deviation of the line-ofsight to thetarget from the boresight axis of the antenna, said apparatuscomprising:

antenna means for receiving said signal and imparting said off-boresightdeviation modulation including means for moving the direction of maximumsensitivity of said antenna means,

means for changing the rate of moving said direction of maximumsensitivity and thereby imparting a frequency change to said deviationmodulation,

and detector means responsive to both the modulation on said receivedsignal and to the changing rate of moving said direction of maximumsensitivity for developing said deviation measurement,

whereby the correspondence between said deviation measurement and theimparted deviation modulation is maximized due to the correlation of thesaid imparted Vfrequency change and said deviation modulation and thesaid effects of various extraneous modulation are lminimized due tononcorrelation with said imparted frequency change.

3. Apparatus as set forth in claim 2 wherein said means for changing therate of moving said direction of maximum sensitivity includes means forgenerating a coded control signal which is applied to said means formoving the direction of maximum sensitivity.

4. Apparatus as set forth in claim 3 wherein said means for generating acoded control signal includes means for changing the frequency of saidcontrol `signal pseudorandomly.

5. Apparatus as in clai-m 4 where the means for changing the frequencyof the pseudo-random control signal includes means for generating -asignal which varies in amplitude pseudo-randomly and oscillator meansresponsive to said amplitude varying signal to generate said frequencychanging signal.

6. Apparatus as set forth in claim 2 including means responsive to saiddeviation measurement for reducing said deviation.

References Cited UNITED STATES PATENTS 2,969,541 1/1961 Seaman 343-120RODNEY D. BENNETT, Primary Examiner.

R. E. BERGER, Assistant Examiner.

U.S. Cl. X.R. 343-100, 117, 118

Disclaimer 3,^H9,8G7.-Petm M. Pz'fer, Atlanta, Ga. AUTOMATIC TRACKINGSYS- TEM UTILIZING CODED SCAN RATE. Patent dutd Dec. 31, 1968.Disclaimer ilcd Aug. 18, 1975, by the assignee, Scientific-Atlanta, Ino. Hereby disclaims the entire term of said patent. [Official Gaaf/ttaFebwlmy I0, 1.976.]

